Many functions of modern devices in automotive, consumer and industrial applications such as converting electrical energy and driving an electric motor or an electric machine rely on semiconductor devices. It is often desirable that the semiconductor devices operate reliably over a long period. A long term high reliability of semiconductor devices is also often expected in consumer electronics, for example in high fidelity audio amplifier circuits. The characteristics of power semiconductor devices such as power transistors used in the amplifier circuit affects the performance of the circuit. It is therefore often desired to prevent or at least delay any degradation of the characteristics such as threshold voltage, blocking voltage, switching time, switching characteristics or amplification.
In particular power semiconductor devices are typically exposed to high loads during operation. For example, a power semiconductor device such as a power IGBT (Insulated Gate Bipolar Transistor) operating in a power converter or as a driver or switch of an electric motor may be exposed to high currents while sweeping-out the excess charge and/or voltages during switching or an operating cycle. In such an event, hot charge carriers, typically hot electrons may be generated in high electric field regions. However, when the hot carriers are injected into a dielectric layer or a field dielectric of the IGBT, degradation of transistor characteristics and even complete circuit failure may occur.
This is not restricted to the active area of power semiconductor devices. Hot carrier injection has also been found to be a reliability risk for edge termination structures in power semiconductor devices. The observed drift of the blocking capability has been attributed to hot electrons which are injected to the dielectric region of edge termination field plates. As the likelihood of hot carrier induced degradation increases with decreasing device dimension, hot-electron-induced degradation is also known to impose limits on the scaling of dielectrics.
In addition, hot-electron-induced degradation of semiconductor devices can often only be detected in sophisticated long-term reliability tests such as high temperature reverse bias tests.
With appropriately poled field plates and/or doped regions the field strength close to the dielectric regions may be reduced. These measures are however not always feasible and imposes design restrictions. For example, using an additional n-doped semiconductor region below a p-doped body region of a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) or IGBT results in a reduced blocking voltage.
For these and other reasons there is a need for the present invention.